Overload sensing tool holders have been proposed and have been of either of two types, either torque or thrust overload. U.S. Pat. Nos. 3,299,697 and 4,193,720 disclose structures responsive to a torque overload. The former patent shows a friction clutch which transmits enough torque to have a gear commence rotating upon torque overload and this gear rotation is sensed by an electromagnetic pickup. The latter patent has interengaging cam surfaces held together by a torsion spring, and upon torque overload, the cam surfaces relatively rotate to axially move a central rod actuating a miniswitch.
The thrust overload devices have been numerous, including those devices shown in U.S. Pat. Nos. 3,381,550; 3,566,719; 3,627,437; 3,652,099; 3,723,017; 3,816,015; 4,090,802; 4,115,025; and 4,260,305. In these devices, an axial thrust overload moves a part on the tool holder axially rearwardly to uncover a radioactive ring sensed by a Geiger counter, to actuate a radiotransmitter, or to actuate a proximity switch or other switch.
Some of these patented devices have been placed on the market, but many are too insensitive, cannot be adjusted readily, are too complicated, or are not versatile enough to be able to be used with all types of tools. For example, with a tool holder carrying a tap which advances toward a workpiece only to find that no hole has been previously drilled for the tap, the thrust overload units would be satisfactory, but a torque overload unit would not be. If, on the other hand, a counterboring tool is performing an undercutting operation in a hole, and the tool is dull, then the torque overload unit could possibly sense this worn tool, but a thrust overload device would be useless.
Accordingly, the problem to be solved is how to construct an overload sensing tool holder which is economical, easy to adjust, and versatile for either torque or thrust overload, or both.